ON-LINE MEASUREMENT OF CRYSTAL SIZE DISTRIBUTION DURING BATCH CRYSTALLIZATION

摘要

The number of crystals and their respective chord length distributionsrnwere measured during batch crystallization using a Lasentec M500Prnon-line particle size analyser. The material used in the study was anrnacidic aromatic compound which had a molecular structure containingrntwo benzene rings. The compound is called C17 in this work. Thernsolubility curve of C17 in water was found to be strongly non-linearrnover the studied temperature range. Therefore, control of batchrncrystallization is difficult. Due to the resulting high supersaturation therncompound tends to form long, needle-shaped crystals, thus causing thernfinal crystal size to be too large. The results show that it is possible tornobtain the desired crystal size and shape by seeding. Crystals ground tornless than 40 mm in size to obtain a sufficient surface area were used asrnseeds. A three-litre U-profiled mixing tank equipped with four bafflesrnand a three-bladed anchor agitator was used. Cooling was performedrnusing different linear and natural cooling rates. The on-line particlernnumber measurements showed that a large number of small nuclei wasrnformed during a short period after seeding. After the initial nuclei werernformed, the number of crystals in the tank did not change notably, if arnsufficient number of seed crystals were used. Furthermore, the meanrnchord length of the crystals attained a constant value after a certainrntime period. The results show that a 1% mass ratio of ground seeds tornproduct crystals is sufficient to fulfil the requirements of the finalrnproduct, assuming that a shower of initial breeding is created in thernbeginning of the batch.